1. Field of the Invention
The present invention relates to a band pass filter and a duplexer.
2. Description of the Related Art
Band pass filters and duplexers are widely used in, for example, cellular phones.
In an elastic wave filter apparatus disclosed in International Publication No. WO 2011/061904, a ladder filter and a longitudinally coupled resonator elastic wave filter are connected in series to each other. The ladder filter includes a plurality of parallel arm resonators. All of the parallel arm resonators in the ladder filter are connected in common to a ground electrode connected to a ground potential. The longitudinally coupled resonator elastic wave filter is connected to a ground electrode different from the ground electrode to which the above-described parallel arm resonators are connected.
In the elastic wave filter apparatus disclosed in International Publication No. WO 2011/061904, since all of the parallel arm resonators in the ladder filter are connected in common to the ground electrode, they are susceptible to a parasitic inductance. Accordingly, the resonance frequencies of the parallel arm resonators sometimes become low. It is therefore impossible to sufficiently increase the steepness on a lower-frequency side of a passband of the elastic wave filter apparatus.
In addition, in the longitudinally coupled resonator elastic wave filter, it is sometimes impossible to make the potential of an electrode connected to the ground electrode sufficiently close to a ground potential. This leads to the degradation of isolation characteristics.